1. Field of the Invention
The present invention relates to a position control apparatus in a machining center or other numerical control machine tool and a method of the same. More specifically the invention is a position control apparatus for correcting positioning deviations occurring in directions other than a positioning direction due to looseness and other mechanical error of a controlled object.
Further, the present invention relates to a numerical control program preparation apparatus for preparing a numerical control program for running a numerical control console for controlling the driving, for example, a machining center or other numerical control machine tool and a method of the same.
Further, the present invention relates to a method of control of a numerical control machine tool for controlling the driving of a numerical control machine tool.
2. Description of the Related Art
In a machining center, lathe, or other numerical control machine tool, the rotational movement from a servomotor or other rotational drive source is converted to linear movement. The conversion an be accomplished by a feed mechanism comprised of a rack and pinion or, ground ball-nut lead screw. The rotational movement is for moving a work table that clamps a workpiece, a tool for cutting the workpiece, or to a predetermined position.
In recent years, use has been made of a numerical control machine tool having a plurality of control axes, such as five axes, for machining three-dimensional free-formed surfaces of workpieces.
As the system for control of the control axes of such a numerical control machine tool, the so-called full-closed feedback system and semi-closed feedback system are known. The full-closed feedback system is a system for position control by directly setting a linear scale or other position sensor on the work table or other controlled object and feeding back a position signal from the position sensor to the servomotor. With this system, it is possible to directly detect the position of the controlled object, so positioning accuracy of the controlled object is high. With the above system, however, a feed mechanism comprised of a relatively low mechanical rigidity rack and pinion, ground ball-nut lead screw, etc. is interposed between the position sensor and the servomotor, so the frequency of the servo system falls and it is difficult to raise the position loop gain. Accordingly, it is difficult to raise the tracking ability of the servo system.
On the other hand, the semi-closed feedback system is a system for indirect position control of a work table or other controlled object. By mounting a resolver or optical rotary encoder or other rotational position sensor on the servomotor driving the work table, or other controlled object, and feeding back the amount of rotation detected from the rotational position sensor to the servomotor, a system can control the amount of rotation of the servomotor. With this system, there is no feed mechanism comprised of a relatively low rigidity rack and pinion, ground ball-nut lead screw, etc. interposed in the servo loop. Consequently a high natural frequency of the servo system can be obtained and the tracking ability of the servomotor can be increased. The semi-closed feedback system, however, controls the driving of the servomotor in order to indirectly control the position of the work table or other controlled object. When reversing the feed direction in a control axis, mechanical positioning error due to so-called backlash, elastic deformation, etc. present in the ground ball-nut lead screw and other feed mechanisms sometimes occurs. Due to this, positioning error sometimes occurs in the direction of a control axis of the work table or other controlled object. If there is such positioning error in the direction of the control axis of the controlled object, it is difficult to precisely machine the workpiece.
Therefore, the conventional numerical control console for controlling the drive of a numerical control machine tool has a function of correcting positioning error called a "backlash compensation function" for each axis. This "backlash compensation function" measures the difference between a position command (target position) and an actual position of the controlled object by a laser measuring device or other detecting means in advance and adding to the position command a predetermined compensation amount to cancel out the above positioning error in a reverse direction to the feed direction of the control axis. By using the above backlash compensation function, it generally becomes possible to accurately control the position in the direction of the control axis even if backlash or other mechanical error occurs.
In machining centers and other numerical control machine tools having a plurality of control axes, however, there are forces acting between the plurality of control axes; positioning errors sometimes occur in other directions at predetermined control points of the tool, work table, or the like even if correcting the axes by the backlash compensation function.
For example, in a machining center having feed mechanisms in the mutually perpendicular X-, Y-, and Z-axis directions, a workpiece clamped on the table may be machined by a rotary tool attached to a spindle.
When cutting a workpiece by a rotary tool while controlling the feed in a predetermined axial direction, for example, the X-axis direction, the X-axis direction feed mechanism is subject to a force caused by the feeding in the X-axis direction, a reaction force, moment, etc. caused by the cutting force, etc. That is, the guide parts of the X-axis direction feed mechanism is subjected to forces acting in the Y- and Z-axis directions in addition to the force in the X-axis direction.
Normally, there are small clearances and other mechanical error between the moving parts of the feed mechanisms and the guide parts of the feed mechanisms. Further, elastic deformation may occur due to the weight of the numerical control machine tool at the guide parts of the feed mechanisms. Accordingly, if forces act in the Y- and Z-axis directions on the guide parts of the X-axis direction feed mechanism, positioning deviation occurs in not only the X-axis direction, but also the Y- and Z-axis directions.
The positioning deviation in the Y- and Z-axis directions caused in the feeding in the X-axis direction feed mechanism appears as remarkable positioning error in the Y- and Z-axis directions since when the X-axis feed direction is reversed, the direction of force acting on the guide parts of the X-axis direction feed mechanism is reversed.
This positioning error occurs in the same way in the Y- and Z-axis direction feed mechanisms and is caused by the forces acting among the X-, Y-, and Z-axis direction feed mechanisms.
In this way, in the related art, even if backlash and other mechanical error occurring in the different control axis feed mechanisms are corrected by the X-, Y-, and Z-axis direction backlash compensation functions, there was still the above positioning error. The existence of this positioning error has been a cause of shape error in the machined surface of workpieces.
This positioning error can occur in the case of a control system using any of the above semi-closed feedback system or the full-closed feedback system.
For example, when using a ball end mill or other rotary tool mounted along the Z-axis direction for cutting while controlling the feed in the X-axis direction, if positioning deviation occurs from the target position in the Z-axis direction in the workpiece and rotary tool, a large cutting load will act on the ball end mill or other rotary tool, the replacement time of the ball end mill will be quickened, and the machining work will have to be frequently stopped. As a result, the machining time will become longer.
On the other hand, if cutting only when feeding in one direction of the X-axis during the finishing to eliminate the problem caused by variations in the position in the Z-axis direction, the machining time becomes about double that compared with reciprocating cutting when feeding in both directions of the X-axis and the machining time therefore is increased.
Further, the numerical control program for cutting in only the X-axis feed direction becomes complicated and therefore a long time and considerable effort are required for preparation of the numerical control program.